The present invention relates to an orthogonal frequency division multiplexing (hereinafter abbreviated to OFDM) demodulator and more particularly a circuit for performing carrier synchronization.
Recently, in the ground-wave digital television broadcasting and the wireless local area network (LAN), an OFDM method which is highly resistant to the multipath interference is used. In the OFDM method, transmission digital data is broken up into a plurality of orthogonal carrier waves to be modulated so that communication is performed.
The modulated OFDM waves in the OFDM method are composed of a plurality of sub orthogonal carrier waves and the sub carrier waves are named sub carriers. Each sub carrier is modulated by the modulation method such as the phase shift keying (PSK) and the quadrature amplitude modulation (QAM). In this manner, in the OFDM method, data is transmitted in parallel by a large number of orthogonal sub carriers and accordingly the time length of symbol can be made long. Moreover, as shown in FIG. 2, the OFDM symbol includes a guard period named a cyclic prefix (CP). The cyclic prefix is a copy of end part of effective symbol so as to maintain the continuity of the waveform in the boundary between the cyclic prefix and the effective symbol and if the delay time of multipath delayed waves comes into the length of the cyclic prefix, interference does not occur between front and rear symbols.
Furthermore, in the OFDM method, since data is multiplexed to a large number of sub carriers, it is general that the inverse fast Fourier transform (IFFT) is performed on the transmission side and the fast Fourier transform (FFT) is performed on the reception side. An FFT window shown in FIG. 2 represents the operation range and the start position of the FFT operation is selected from any position from the top to the end of the cyclic prefix. Since the cyclic prefix is a copy of end part of the effective symbol so as to maintain the continuity of the waveform in the boundary of the cyclic prefix and the effective symbol, the orthogonality among the sub carriers can be maintained as far as the start position of the FFT operation is within the operation range.
In order to demodulate the modulated OFDM waves by the OFDM demodulator, synchronization of every kind is required. Particularly, synchronization of the symbol timing for detecting the boundary position of the OFDM symbol and synchronization of the carrier frequency for synchronizing carrier frequency of the modulated OFDM waves with a carrier frequency for reproduction used on the reception side are required and a number of methods have been proposed until now.
For example, U.S. Pat. No. 5,608,764 (Japanese Patent No. 3041175) (patent document 1) discloses the method of performing clock synchronization without using a reference signal for the clock synchronization. In this method, constellation is analyzed on the basis of the result of the FFT operation and an error in the carrier frequency for orthogonal demodulation and an error in frequency of a sampling clock for analog-to-digital (A/D) conversion are judged and corrected from the phase rotation direction of 2 or more different sub carriers.